1. Field of the Invention
The present invention relates to a wireless communication system that adopts a closed-loop transmit diversity technique, and more particularly to an antenna verification carried out with a wireless receiving apparatus.
2. Description of Related Art
The diversity technique is promising as a technique capable of suppressing deteriorations in transmission quality of wireless communications, especially, mobile communications under the fading environment. As an example of the diversity technique, there is a transmit diversity technique. The transmit diversity technique is such that a wireless transmitting apparatus transmits signals from plural antennas, and a wireless receiving apparatus receives the transmission signals with a one-antenna. Upon the transmit diversity, the wireless transmitting apparatus generally needs to grasp a state of a transmission path to the wireless receiving apparatus prior to the signal transmission. A technique for grasping the transmission path condition based on feedback information from the wireless receiving apparatus is hereinafter referred to as “closed-loop transmit diversity”.
For example, even 3GPP (3rd Generation Partnership Project) specifications as the W-CDMA (Wideband-Code Division Multiple Access) communication system standards adopts two closed-loop transmit diversity schemes: “mode 1” and “mode 2” (see Japanese Unexamined Patent Application Publication No. 2003-8552, for example) . Among these, the closed-loop mode 1 implements the transmit diversity by use of two transmission antennas (hereinafter referred to as “first antenna” and “second antenna”) in a wireless transmitting apparatus of a base station. A mobile station sends to base station, feedback information for controlling a phase of a transmission carrier from the second antenna so as to maximize received power of signals sent from the base station. In the closed-loop mode 1, an appropriate one is selected from four offset phase angles applied to a transmission carrier from the second antenna.
In the closed-loop mode 2 as well, similar to the closed-loop mode 1, the transmit diversity is implemented using two transmission antennas in the wireless transmitting apparatus of the base station. However, the mode 2 differs from the mode 1 in that an appropriate amplitude can be selected in addition to the phase angle of the transmission carrier from the second antenna, based on the feedback information sent from the mobile station. In the closed-loop mode 2, an appropriate one is selected from 8 phase angles of the transmission carrier from the second antenna, and an appropriate transmission carrier amplitude is selected from two amplitudes to thereby select a desired combination of phase and amplitude from 16 patterns.
Brief explanation is given of a conventional W-CDMA communication system that implements the transmit diversity in the closed-loop mode 1. FIG. 3 shows the configuration of a base station side wireless transmitting apparatus 5, and FIG. 4 shows the configuration of a mobile station side wireless receiving apparatus 7. First, the configuration of the wireless transmitting apparatus 5 of FIG. 3 is described. A channel encoding unit 51 receives a Dedicated Channel (DCH) data sequence as a transmission data sequence to execute channel encoding and bit-interleaving, and then multiplexes the channel data with individual pilot bits and control information such as a TPC (Transmit Power Control) command to generate a Dedicated Physical Channel (DPCH) . Incidentally, the channel encoding unit 51 outputs data of two DPCHs (DPCH 1 and DPCH 2) to be sent from a first antenna 63 and a second antenna 64.
An antenna weight generator 52 generates weight vectors w1 and w2 to be multiplexed with the two DPCHs generated with the channel encoding unit 51 based on FBI (Feedback Information) bits from an FBI bit determining unit 53. The weight vectors w1 and w2 are multiplexed with the data of two DPCHs by the multipliers 55 and 56, making it possible to rotate a transmission carrier phase and give a phase offset angle to signals sent from the two antennas 63 and 64.
In the closed-loop mode 1, the phase offset angle φ between the transmission signal of the first antenna 63 and the transmission signal of the second antenna 64 is π/4, −π/4, 3π/4, or −3π/4, and a combination of the weight vectors w1 and w2 is one of the following four patterns represented by Expression 1.(w1, w2)=(1, exp(jφ)) φε{π/4, −π/4, 3π/4, −3π/4}  Expression 1
The FBI bit determining unit 53 determines an FBI bit value received from the mobile station. Here, the FBI bit means information for controlling the transmission carrier phase of the DPCH sent from the second antenna 64 such that the signals from the two antennas 63 and 64 of the wireless transmitting apparatus 5 are almost in phase at the receiving unit of the mobile station. The FBI bit is generated by the wireless receiving apparatus 7 as described below, and sent to the base station side wireless transmitting apparatus 5 by use of the up DPCH.
Incidentally, the closed-loop mode 1 defines the number of FBI bits in the up DPCH to 1 bit per slot of the DPCH. Thus, one of the above four phase offset angles is designated based on FBI bits corresponding to 2 slots of the DPCH. Provided that an FBI bit for even-numbered slots of the up DPCH is represented by FBI_e, and an FBI bit for odd-numbered slots is represented by FBI_o, the phase offset is defined based on the latest combination (FBI_e, FBI_o) as follows:
If (FBI_e, FBI_o)=(0,0), φ=π/4
If (FBI_e, FBI_o)=(0,1), φ=−π/4
If (FBI_e, FBI_o)=(1,0), φ=3π/4
If (FBI_e, FBI_o)=(1,1), φ=−3π/4
A spreading code generator 54 generates spreading codes for the two DPCHs. The multipliers 57 and 58 multiplex the spreading codes generated by the spreading code generator 54 with data of the two DPCHs.
A multiplexing unit 59 multiplexes the data of DPCH 1 to be sent from the first antenna 63 and data of a common pilot channel CPICH 1 to transmit the composite one to a transmitting unit 61. A multiplexing unit 60 synchronizes the data of the DPCH 2 to be sent from the second antenna 64 and data of a common pilot channel CPICH 2 to transmit the composite one to a transmitting unit 62. Here, the common pilot channel CPICH is a channel for transmitting a pilot symbol, and the CPICH 1 and the CPICH 2 are spread by use of the same spreading code, and orthogonalized by changing symbol patterns of the pilot signals.
The transmission signals multiplexed by the multiplexing units 59 and 60 are subjected to D/A conversion, quadrature modulation, and frequency conversion into an RF signal, and signal amplification at the transmitting units 61 and 62 and then transmitted from the first antenna 63 and the second antenna 64, respectively.
The configuration of the wireless receiving apparatus 7 of FIG. 4 is described next. A receiving unit 12 receives a reception signal from an antenna 11 to execute signal amplification, frequency conversion (down conversion), and orthogonal detection to obtain in-phase analog signal components and orthogonal analog signal components. The receiving unit 12 further executes A/D conversion on the in-phase analog signal components and the orthogonal analog signal components to output the converted ones to a DPCH despreading unit 13 and a CPICH despreading unit 14.
The DPCH despreading unit 13 despreads the components using the same spreading code as the spreading code used for the DPCH in the base station side wireless transmitting apparatus 5. The CPICH despreading unit 14 despreads the components using the same spreading code as the spreading code used for the CPICH in the wireless transmitting apparatus 5.
A phase comparator unit 15 calculates a phase difference by comparing the phase of the CPICH 1 sent from the first antenna 63 with the phase of the CPICH 2 sent from the second antenna 64. A FBI bit generating unit 16 determines a phase offset of the CPICH 2 sent from the second antenna 64 so as to minimize a phase difference between the CPICH 1 and the CPICH 2, and gives the FBI bit corresponding to the determined phase offset to the up DPCH to transmit the obtained one to the base station.
Incidentally, the number of FBI bits is set such that 1 bit is assigned to 1 slot of the up DPCH as described above. Thus, in the case of even-numbered slots, the FBI bit generating unit 16 determines whether the phase offset angle φ is 0 or π. If it is determined that φ=0, FBI_e=0. On the other hand, if it is determined that φ=π, FBI_e=1. Further, in the case of odd-numbered slots, the FBI bit generating unit 16 determines whether the phase offset angle φ is π/2 or −π/2. If it is determined that φ=π/2, FBI_o=0. On the other hand, if it is determined that φ=−π/2, FBI_o=1.
A first antenna channel estimation unit 17 estimates characteristics of a channel of the signal sent from the first antenna 63 to complement phase variations due to fading. A RAKE combination unit 22 described later combines plural multipath components. The respective multipath components are received through different channels and thus are subjected to different fading environments and differ from each other in terms of phase variations due to the fading. To that end, in order to execute synchronous detection, it is necessary to estimate the degree of phase variation of each path due to the fading to complement the variations.
In general, in the W-CDMA communication system that adopts the closed-loop transmit diversity scheme, a channel is estimated using a pilot symbol in the CPICH. The CPICH is a channel for transmitting the pilot symbol of a fixed pattern, and its transmission phase is known to the wireless receiving apparatus 7. Thus, a first antenna channel estimation unit 17 compares the transmission phase with a reception phase of the CPICH 1 which is sent from the first antenna 63 similar to the DPCH 1 to reach the wireless receiving apparatus 7 by way of the same channel as that for the DPCH 1 to obtain an estimate of the phase variations due to the fading. A multiplier 20 multiplexes a complex conjugate of the estimated values obtained by the first antenna channel estimation unit 17 with a data symbol of the DPCH from the DPCH despreading unit 13 to thereby complement the phase variations due to the fading.
A second antenna channel estimation unit 18 estimates characteristics of a channel of the signal sent from the second antenna 64 to complement phase variations due to fading. The second antenna channel estimation unit 18 compares the reception phase of the CPICH 2 for transferring a pilot symbol with the transmission phase similar to the first antenna channel estimation unit 17 to obtain an estimate of the phase variations due to the fading. Here, a phase offset is added to the DPCH 2 sent from the second antenna 64. Therefore, the second antenna channel estimation unit 18 needs to receive phase offset information from an antenna verification unit 79 to carry out channel estimation in consideration of the phase offset.
The antenna verification unit 79 receives the signals of the reception CPICH 2 and the reception DPCH 2 to verify the phase offset applied to the DPCH 2 by the base station side wireless transmitting apparatus 5. Here, the antenna verification means that the wireless receiving apparatus that receives the transmission signal based on the closed-loop transmit diversity scheme estimates a weight vector that is added to the transmission signal by the wireless transmitting apparatus.
As described above, in the transmit diversity scheme of the closed-loop mode 1, the DPCH 2 from the second antenna is given a phase offset as the weight vector. If an bit error occurs in the FBI bit sent from the mobile station to the base station, a phase offset that is given to the DPCH 2 by the base station is different from a phase offset that is designated by the mobile station based on the FBI bit. Thus, it is necessary to verify the phase offset applied by the base station through the antenna verification.
For example, in the closed-loop mode 1, the wireless receiving apparatus 7 of the mobile station verifies the phase offset that is applied to the DPCH 2 from the second antenna 64 by the wireless transmitting apparatus 5. The verification result is used for channel estimation at the second antenna channel estimation unit 18.
There are some antenna verification methods, one of which is described in 3GPP TS25.214 AnnexA. Hereinbelow, description is made of a method of verifying a phase offset applied to the down DPCH 2 by the base station while verifying 1 bit of the FBI bit for each slot.
Incidentally, the following description is given on the assumptions that FBI bits sent by the mobile station is 0 all the time and that the antenna verification is executed on the FBI bits of the even-numbered slots, i.e., FBI_e, no bit error is found in FBI bits of the odd-numbered slots as previous slots, which are received by the base station, and it is determined that FBI_o=0 at the base station.
If the base station receives the FBI bit with no bit error, an expected phase value to be added to the down DPCH2 is defined as a first candidate phase value, and if the base station receives the FBI bit with any bit error, an expected phase value to be added to the down DPCH2 is defined as a second candidate phase value. In other words, the first candidate phase value is π/4 radians since (FBI_e, FBI_o)=(0,0), and the second candidate phase value is 3π/4 radians since (FBI_e, FBI_o)=(1,0).
The down CPICH 2 and the down DPCH 2 sent from the second antenna 64 reach the wireless receiving apparatus 7 through the same channel, so the degree of phase variation in the DPCH 2 due to the fading is equivalent to that in the CPICH 2. The phase of the DPCH 2 is further rotated by a phase offset. Assuming that a complex vector that represents an influence of the fading is β, the DPCH 2 and the CPICH 2 influenced by the fading are represented by β (DPCH 2) and β (CPICH 2), respectively, β (CPICH 2) ideally becomes in phase with β (DPCH 2) after rotated by an angle corresponding to a phase offset applied to the DPCH 2.
Based on the above properties, the antenna verification verifies a phase offset applied by the base station by determining which one of the complex vector phase obtained by rotating the phase of β (CPICH 2) by the first candidate phase value and the complex vector phase obtained by rotating the phase of β (CPICH 2) by the second candidate phase value is closer to the phase of β (DPCH 2). Incidentally, the phase of β (DPCH 2) may be derived from the dedicated pilot bit multiplexed with the DPCH 2 and having a known symbol pattern.
To be specific, first, a first replica R1 of β (DPCH 2) is prepared by rotating a phase of β (CPICH 2) by the first candidate phase value, π/4 radians, and a second replica R2 of β (DPCH 2) is prepared by rotating a phase of β (CPICH 2) by the second candidate phase value, 3π/4 radians. The replicas R1 and R2 are represented by Expressions 2 and 3 below.R1=β(CPICH2)×exp(jπ/4)  Expression 2R2=β(CPICH2)×exp(j3π/4)  Expression 3
Next, the correlation degree S1 between the first replica R1 and β (DPCH 2) and the correlation degree S2 between the second replica R2 and β (DPCH 2) are determined and compared with each other to thereby select a replica having a higher degree of correlation with β (DPCH 2), and a candidate phase value corresponding to the selected replica is determined as a phase offset applied on the base station side. Incidentally, the correlation degrees S1 and S2 may be other criteria for judgment, for example, the vector inner product between the replicas and β (DPCH 2) as long as the phase differences between the replicas R1 and R2, and β (DPCH 2) can be compared.
As a result of the above judgment, if the second candidate phase value is determined as the phase offset, the judgment result shows that a bit error occurs in the FBI bits received by the base station. To that end, it is necessary to update the value of the FBI bit (FBI_e) of the even-numbered slot for verification from 0 to 1 upon the antenna verification for a subsequent odd-numbered slot. In the case of updating FBI_e, the first candidate phase value for the subsequent odd-numbered slot is 3π/4 radians since (FBI_e, FBI_o)=(1,0), and the second candidate phase value is −3π/4 radians since (FBI_e, FBI_o)=(1,1).
FIG. 5 shows a processing flow example of the above antenna verification method. In step S501, the correlation degree S1 between β (DPCH 2) and the first replica R1 added with the first candidate phase value as the phase offset is calculated. In step S502, the correlation degree S2 between β (DPCH 2) and the second replica R2 added with the second candidate phase value as the phase offset is calculated. In step S503, the correlation degrees S1 and S2 are compared to select a phase offset with the higher correlation degree.
In step S504, it is determine whether or not the selected phase offset is the second candidate phase value. In the case of selecting the second candidate phase value, the bit error occurs in the FBI bit received by the base station, so the FBI bit value for subsequent antenna verification is updated (in step S505). That is, as described above, in the case of selecting the second candidate phase value upon the antenna verification for the even-numbered slot, the FBI bit value, FBI_e of the even-numbered slot is updated for verification of a subsequent odd-numbered slot. In step S506, the complex conjugate of the selected phase offset is output to the multiplier 21.
Referring back to FIG. 4, the RAKE combination unit 22 executes RAKE combination of multipath components the phase variations of which are complemented by the first antenna channel estimation unit 17 and the multiplier 20. Likewise, the RAKE combination unit 23 executes RAKE combination of multipath components the phase variations of which are complemented by the second antenna channel estimation unit 18 and the multiplier 21. Output values of the RAKE combination units 22 and 23 are added with an adder 24 and then input to a channel decoding unit 25. The channel decoding unit 25 deinterleaves and channel-decodes the input data sequence to output decoded data.
In the antenna verification carried out by the conventional wireless receiving apparatus 7, if it is erroneously determined that a bit error occurs in the FBI bit received by the base station, and a wrong phase offset is determined as a phase offset added to the DPCH2 by the base station, it is impossible to accurately perform channel estimation for the slot on which erroneous judgment is made. Further, upon the antenna verification for a slot subsequent to the slot concerned, the first candidate phase value and the second candidate phase value are generated using the FBI bit value updated on the basis of the erroneous judgment result. That is, if an erroneous judgment is made upon the antenna verification for a previous slot, none of the candidate phase values thereof match a phase offset set by the base station. Thus, if either the first candidate phase value or the second candidate phase value is selected through the antenna verification for a slot subsequent to the slot on which the erroneous judgment is made, correct channel estimation cannot be performed.
For example, in the case where an erroneous judgment is made upon the antenna verification for the even-numbered slot, the FBI bit value, FBI_e is erroneously updated to “1” upon the antenna verification for a subsequent odd-numbered slot. As a result, the first candidate phase value at the time of verification for the subsequent odd-numbered slot is 3π/4 radians since (FBI_e, FBI_o)=(1,0), and the second candidate phase value is −3π/4 radians since (FBI_e, FBI_o)=(1,1). However, the phase offset set by the base station is π/4 radians based on the FBI values (FBI_e, FBI_o)=(0,0), so none of the candidate phase values match the correct phase offset.
As described above, in the conventional antenna verification method, if an erroneous judgment is made upon the verification at a given timing (a given slot in the case of W-CDMA), the channel estimation cannot be accurately carried out not only at that timing but also at a subsequent timing when a candidate value of the phase offset is determined based on the result of antenna verification that reflects the result of the erroneous channel estimation.
As described above, the conventional antenna verification method has a problem that if an erroneous antenna verification is made at a given timing, this error influences channel estimation based on the result of subsequent antenna verification.
Incidentally, such problems arise not only in the closed-loop mode 1 of the above W-CDMA but also in other wireless communication systems that adopt a closed-loop transmit diversity scheme upon the antenna verification.